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Temperature confusion

OK, I’m fairly new to brewing, but I’m pretty sure the laws of physics still apply. Last brew day while chilling the wort, I used a floating thermometer to let me know when it was time to transfer and pitch. It read about 80F and I was about to start siphoning when I checked it with one of those little instant read thermometers from NB, which read 130F. OK, maybe the first one was floating too close to the wort chiller. But moving things around and trying different angles still produced a 40ish degree difference. later I stuck both in the snow, and after 10 minutes there was a 10 degree difference.

So, Is it possible for a standard thermometer to be off, or does the digital need to be calibrated, and if so, how

It is possible for any thermometer to be off, though it happens to digital thermometers more frequently than liquid-in-glass ones. A liquid in glass thermometer is very reliable if it was calibrated properly when it was made. You will pay a lot to get one that was certified as being properly calibrated, the cheap ones can easily be off by several degrees right out of the factory. It is possible for a bubble to get into the liquid (I’ve seen this on rare occasions) and throw the readings off, but it is pretty easy to see that.

Digital thermometers should be calibrated as per the manufacturer’s recommendations - they vary a lot.

Is the instant read one that doesn’t actually touch the liquid? An IR thermometer? If yes, I would immediately suspect it of being the one that is off.

The quickest test you can do to figure out which one is accurate is try putting both in a jar of water along with a third one. If two of the thermometers agree, it is very likely that they are the ones telling the correct temperature.

http://www.foodsafety.unl.edu/haccp/hel ... alibration

50° difference sounds like way too much unless there is something drastically wrong with one of your thermometers. Did you stir the wort before reading the temp? I can read three different spots in my mash tun and get three different readings. I also have two thermometers in it that always read different. After checking it with another one I determined which one is off and don’t use it.[quote=“Hades”]http://www.foodsafety.unl.edu/haccp/helpful/thermcalibration[/quote] Hades link has good info on how to test your thermometers. If you determine how far one is off you don’t necessarily have to toss it as long as you know how much to adjust the reading.

Make sure you are stirring while chilling, you will chill faster and have a more even temperature. The wort chiller is only chilling the wort that is touching it.

I had horrible luck with this one. http://www.northernbrewer.com/shop/proa … meter.html It worked a few times then went nuts.

This one however was really solid for me http://www.northernbrewer.com/shop/km14 … meter.html It was down graded to food usage after I got a thermopen ( a 100 dollar thermometer) but every time I’ve checked it was dead on with thermopen. It just takes longer to get a reading.

the second link is the digital thermometer I have. Thanks for the link hades, I will try the boiling method and repost which one was so horribly off, assuming its not both of them

No problem. I’d actually do boiling and ice water tests just to rule out high/low variances.

I just bought a dial thermometer and it says for recalibration use a “slush ice solution (60% crushed ice and 40% tap water) and adjust pointer to 32 degrees after minute for sensor to reach equilibrium.” I think ice water is the best way because it doesn’t vary where as boiling water temp will vary with altitude and barometric pressure.

Not exactly true. Salt concentrations will change the freeze point, so to actually get an ice-water mix at 32, you should be using distilled water and ice made from distilled water. And the difference can be pretty big - 0 F was originally defined as the temperature at which blood (or sea water depending on which story you read) freezes.

But in general, yes, using an ice-water mix is pretty reliable for knowing if your thermometer is accurate at 32F. The problem is that as brewers, we want to know the temperature accurately at 50-70F, and at 140-170F. And errors in thermometer readings are not necessarily constant or even linear with temperature.

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